A tire is a pneumatic system, which supports a vehicle’s load. It does this by using a compressed gas (usually air) inside to create tension in the carcass plies. It’s important to realize that a tire carcass has a high-tension strength, but has little or no compression strength. It’s the air pressure that creates tension in the carcass and allows the tire to function as a load-carrying device.

That's why inflation is so important. In an inflated, but unloaded tire, the cords pull equally on the bead wire all around the tire. When a tire is loaded, the tension in the cords between the rim and the ground is balanced or relieved. The tension in other cords is not changed. Therefore, the cords opposite the ground pull upwards on the bead. This is the mechanism that transmits the pressure from the ground to the rim.

In addition, a tire must transmit handling (acceleration, braking, cornering) to the road. Cornering forces are transmitted to the rim in a similar manner to load. Acceleration and braking forces rely on the friction between the rim and the bead. Inflation pressure also supplies the clamping force, which creates friction.A tire also acts as a spring between the rim and the road. This spring characteristic is very important to the vehicle’s ride. Too high an inflation pressure causes the tire to transmit shock loads to the suspension and reduces a tire's ability to withstand road impacts. Too low an inflation pressure reduces a tire's ability to support the vehicle's load and transmit cornering, braking and acceleration forces.

Inflation Problems

Under-inflation can cause many tire-related problems. Since a tire’s load capacity is largely determined by its inflation pressure, under-inflation results in an overloaded tire. An under-inflated tire operates at high deflection resulting in decreased fuel economy, sluggish handling and may result in excessive mechanical flexing and heat buildup leading to catastrophic tire failure.

Correct inflation is especially significant to the endurance and performance of radial performance tires. For example, because of a performance radial’s aspect ratio and design, it may not be possible to look at a radial tire and actually see under-inflation of 5 psi. However, under-inflation of 5 psi can reduce a performance tire's tread life by 25%. A typical tire may also lose 1 to 2 psi a month, if not checked and adjusted.

Temperature Effects
Air pressure is affected by temperature. The air under pressure in a tire is no exception. Typically, an inflation pressure can change by 1 psi for every 10 degrees Fahrenheit of temperature change. Higher temperature means increased pressure.

For example, if a tire is inflated to 35 psi on an 80°F July day, it could have an inflation pressure of 23 psi on a 20°F day six months later in January. This represents a normal loss of 6 psi over the six months and an additional loss of 6 psi due to the 60°F temperature change. At 23 psi, this tire is severely under-inflated.All tire makers recommend that consumers check their tire inflation pressure at least once each week, and as often as possible when tires are cold (ambient air temperature and if the vehicle has not been driven for several hours).

Excessive inflation loss (more than 2 psi per week), visible damage and irregular wear, or tires experiencing severe impacts, vibration, or pulling require removal along with an expert inspection of tire and rim to determine reparability or the need for replacement. Damaged tires may fail suddenly or burst upon re-inflation, resulting in serious injury.

Article by Tire Review Magazine.